The present invention relates to a device for determining the arrival time of pulses, principally in the case where these pulses have different rising fronts. The invention also relates to the use of such a device in distance-measuring equipment or DME and the equipment comprising such a device.
In some equipment belonging to the class of electromagnetic detection equipment and in particular for distance-measuring equipment, the pulses used must conform to certain norms and definitions.
FIG. 1 shows the general shape of the pulses used in a DME for which some definitions are given below.
The amplitude of the pulse is the maximum amplitude A of its envelope. The duration of the pulse is the time interval between the points b and g, situated respectively on the leading and trailing edges of the pulse at the 0.5A level.
The rising time of the pulse is the total rising time measured between points a, at 0.1A on the leading edge of the envelope of the pulse and c at 0.9A on the same leading edge.
The quench time of the pulse is the total quench time measured between points e, at 0.9A on the trailing edge of the envelope of the pulse and h, at 0.1A on this same trailing edge.
Since these pulses are used for determining the distance separating a moving object from any point whatsoever, a beacon for example, the time of arrival of such a pulse in the measuring equipment, not only onboard the moving object but also on the ground, must be known accurately and in particular for avoiding the errors which would result from a modification of the shape of the signal received due to parasites. Of course, the measurement of distance is not limited to airborne moving bodies; it may also apply to moving bodies on land or at sea. This determination is particularly important in distance-measuring equipment or DME, wherein it is desired to know the distance between an airborne moving body and a ground station which may be the landing point for the aircraft.
A device for determining the time of arrival of pulses is known, used in a so-called "en route" DME which seeks to find the distance between an aircraft in flight and a station.
This en route DME equipment uses Gaussian-shape pulses with a rising front of the order of 2.5 .mu.s between 10% and 90%, in fact between 1 and 3 .mu.s. The determination or also the detection of the arrival time of a pulse of this type is achieved by means of an amplitude comparator circuit working out the decision at mid-height of the pulse. The prior-art device comprises then a peak detector which takes the peak value of the pulse then applies half of this peak value to the input of an amplitude comparator which receives at the other input the pulse suitably delayed by a delay line. FIG. 2 gives a time diagram of the execution of this measurement, representing the amplitude V of the pulse used with respect to the time t. Curve B shows the pulse used in the equipment which reaches its maximum amplitude A after a time tc, greater than time tr, called rise time, in accordance with the above-recalled norm. Curve C shows the pulse delayed by time td, td being approximately equal to time tc. The arrival time ta of the pulse is marked by the point D, where the pulse reaches half of the peak value, i.e. 1/2A.
However, and principally when it is desired to determine the distance between the aircraft and the ground station with greater accuracy, in the case where the ground station is the landing point of the aircraft, this device is not accurate enough for it becomes sensitive to external parasite phenomena, in fact to the reflections due to the multipaths which cause modifications of the shape of the signal received and this until the peak value of the pulse is obtained, i.e. until switching of the comparator used in the determination device. These modifications in the shape of the curve lead to erroneous positioning of point D.
To remedy these disadvantages, a device has been proposed corresponding to another method for determining the arrival time of pulses and working with pulses having a faster rising front, generally of the order of 1 .mu.s.
A thus improved device is known under the name of DAC (delay and compare), that is to say that the operation of this circuit is based on a delay and a comparison.
FIG. 3 shows such a device and FIG. 4 the pulses used in an amplitude-time diagram.
The device for determining the arrival time of a pulse, improved with respect to the one previously mentioned, comprises a comparator in the form of a differential amplifier 1 one input of which, generally the positive input, is fed from the input E of the device by a circuit comprising a delay line 3 matched to a characteristic resistance 4, of value Ro and the negative input of which is connected to the input E of the device through a circuit comprising a voltage divider formed by a resistor 5, of value R1, in series with a resistor 6, of value R2. A biasing voltage +Vp is applied to the negative input of comparator 1 through a resistor 7 and a diode 8.
With this circuit it is sought to determine the arrival time of the pulse after reception thereof, without waiting for it to reach its peak value. In this case, the so-called detection time is then less than the rise time tr.
Thus, the DAC device of FIG. 3 is much less sensitive to the multipaths than the previously described device and gives better accuracy of measurement, which accuracy is required when the aircraft is in approach and/or landing mode where as it happens the effect of the multipaths may be considerable and harmful.
In this device, the pulse which is applied to input E is shown by the signal B in FIG. 4 which gives the amplitude-time diagram of the different signals used in the device. This pulse is on the one hand attenuated, signal F, by the resistive divider bridge 5-6 (R1, R2) and applied to the negative input of the comparator and on the other hand delayed in line 3 (signal C) by a relatively short time td before being applied to the positive input of the comparator. The crossing point D of the attenuated pulse F and the non-attenuated delayed pulse C gives the arrival time ta of the received pulse.
To avoid the comparator from being triggered by the noise which, because of the attenuation k=R2/(R1+R2), is greater at the non-inverting, i.e. positive, input than at the inverting, i.e. negative, input, there is applied to this latter a protection-biasing voltage V.sub.p whose amplitude is slightly greater than the amplitude of the noise, with the desired protection level. It will be noted that a non-inverting input in a comparator is the one which, receiving a signal of a given sign, corresponds to an output delivering a signal of the same sign.
It has been established that with the DAC device the effect of the parasite reflections due to the multipaths only occurs for a very short time, considerably less than time ta marking the arrival time of the pulse.
This DAC device improves then the problem of determining the arrival time of a pulse by reducing the effect of the multipaths with respect to a circuit of the peak-detector type. For a given rise time of the pulses to be received the parameters of the device, i.e. the delay td and the attenuation k, are chosen accordingly.
It is then necessary to consider how this given DAC device reacts generally for pulses having a rise time less than 2.5 .mu.s when there is applied thereto a pulse having a relatively large rise time, for example of the order of or greater than 2.5 .mu.s. FIG. 5 gives a diagram with respect to time of the response of the DAC device for two pulses having different rise times. Signal C1 is the pulse with a short rise time delayed by time td, signal F1 is the non-delayed attenuated pulse. Signal C2 is the pulse with a long delayed rise time and F2 the corresponding non-delayed attenuated pulse. Points D1 and D2 characterize respectively the arrival times ta of the two pulses and it can be seen that the levels A1 and A2 at which the decision must be taken are different; the level at which the decision is obtained on the slow pulse C2, i.e. A2, is considerably less than level A1 relative to the fast pulse C1.
This result has troublesome consequences when the determination device may or must be used with the two sorts of pulses. This case is that of a so-called precision DME operating in the approach or landing phase of an aircraft and which may nevertheless receive slow pulses of the type used at present in a so-called en route DME, in which accuracy plays a less important role. With the DAC circuit described, the proper operation of the device with a slow pulse requires a signal-to-noise ratio greater than the signal-to-noise ratio of the fast pulse since the level where the determination of the arrival time of the slow pulse C2, i.e. A2, takes place is lower than level A1. There is loss of sensitivity in the case of reception of the pulses with a slow rise time.
Furthermore, the passband of the receiver is calculated for transmission of the fast pulse, i.e. that it is much wider than necessary for the reception of a slow pulse, thus causing a signal-to-noise ratio loss for this pulse.